The present invention relates to a twin belt type casting machine which is capable of freely altering the width of a thin slab produced, and more particularly to a twin belt type casting machine which is capable of preventing the deformation of belts and a method of casting by using the same.
In recent years, a continuous casting method for directly producing from molten metal, such as molten steel, a thin slab having a thickness of several millimeters to several dozens of millimeters and a configuration close to that of a final product has come to be highlighted. According to this method, since it is possible to omit a rolling process involving a multiplicity of stages which have conventionally been used, the processes and facilities can be simplified. In addition, since steps for heating the basic material to a working temperature between the respective processes are essentially unrequired, it is possible to expect an energy-saving effect. Among such continuous casting methods, a twin belt type casting method is known.
In this twin belt type casting machine, the molten metal in a tundish is supplied through a nozzle into a casting space. This casting space is defined by a space formed between a pair of belts which are made of a heat-resisting material such as steel and are respectively adapted to run by being trained among pulleys, the both side portions of the space being respectively partitioned by edge dams. The molten metal poured into this casting space is cooled and solidified by cooling boxes and is discharged as a thin slab.
At this time, if there is any clearance between the belt and the edge dam, the molten metal would enter that clearance, resulting in an outset. For this reason, it is necessary to press the belt against the edge dam.
As for the edge dam, there are the following two types; one is a type which does not move in the direction of movement of the metal being cast, and the other is an endless coupled type edge dam which moves in synchronism with the movement of the metal being cast. Although these two types are generally called an edge dam, when the two types are expressed individually, the former is called a fixed edge dam, and the latter a synchronously moving edge dam.
When the width of the metal being cast is altered by using the fixed edge dams, the edge dams are expanded or shrunk in the transverse direction of the belt, i.e., perpendicularly to the direction of movement of the belt.
Meanwhile, when the width of the metal being cast is altered by using the synchronously moving edge dams, there are the following two methods; one in which the synchronously moving edge dams are expanded or shrunk together with bases thereof in the transverse direction of the belt, and the other in which, when cooling blocks constituting the synchronously moving edge dams are inserted consecutively between the belts in the moving direction thereof, the blocks are inserted after the positions of installation of the blocks are changed to the expanded or shrunk positions in the transverse direction of the belt. In this case, although the blocks themselves do not move in the transverse direction of the belt, the width of the cast piece is varied by the insertion of the group of blocks whose positions have been changed.
Thus, although there are various methods of varying the width of the metal being cast, in the present invention, the movement described in these methods will, for simplicity's sake, be generally referred to as the transverse (widthwise) movement of the edge dams.
The present inventors have developed a mechanism for pressing the edge dams, and filed an application for patent as Japanese Patent Laid-Open Publication No. 61-99541. In this apparatus, the edge dams are disposed such as to be movable in the transverse direction of the belt, and edge dam supporting blocks are also provided in the cooling box in addition to edge dams disposed on both sides of the cooling box. These edge dam supporting blocks are capable of pressing and depressing freely the belts as a driving force of a pressing device is transmitted thereto through a rod. Thus the width of the thin slab can be altered as a plurality of such edge dam supporting blocks are provided in the transverse direction of the cooling box.
Namely, when a thin slab of a maximum width is produced, the both side portions of the casting space are partitioned by the outside edge dam supporting blocks. At this time, a refrigerant is supplied to the entire space between the belt and the cooling box. When a thin slab of a small width is produced, the edge dams are moved to inward locations in the transverse direction of the belt, and the edge dam supporting blocks in those locations are pressed against the belt to form the casting space. In addition, the refrigerant is supplied to a gap between the portion of the belt which is disposed inwardly of the edge dam supporting block in relation to the transverse direction of the belt and the cooling box, while the supply of the refrigerant to a gap located outwardly thereof is stopped. A plurality of ribs are provided on the surface of the cooling box opposed to the belt, and these ribs serve to prevent the belt from approaching excessively to the cooling box by the static pressure of the molten metal and to secure a predetermined channel for the refrigerant.
As the edge dam supporting blocks are thus made movable, it becomes possible to produce a thin slab having a required width by means of the same continuous casting apparatus.
In addition, when the thin slab is being cast, the portion of the overall surface of the belt which is brought into contact with the molten metal is subjected to a large heat flux, the temperature of the belt itself becomes higher in a central portion thereof as compared with the other portions. As a result, the belt undergoes thermal expansion at the central portion thereof, and it is known that this causes the deformation of the belts. This condition is schematically shown in FIG. 26. As a method of preventing this deformation, it is effective to make the temperature of the overall surface of the belts uniform. For instance, in U. S. patent application No. 3,937,270 (Japanese Patent Examined Publication No. 57-61502), it is assumed that the cause of the occurrence of displacement of the belts lies in the cold framing on the entrance side of casting, and as a measure against it a proposal is made therein to make the temperature of the belts higher in advance. In addition, in Japanese Utility Model Unexamined Publication No. 59-58550, a proposal is made to heat the opposite side end portions of the belts so as to set the temperature of those portions at the same level as that of the central portion thereof.
Furthermore, it is proposed in U.S. patent application No. 3937270 (Japanese Patent Examined Publication No. 57-61502) that, when producing cast slabs with different widths by using one type of belt mold, the quantity of water at opposite side end portions of the belts be changed. However, no means has yet been proposed which allows the positions of cooled portions and uncooled portions to be changed by following the change in the width of the thin slab being cast, when the width of the thin slab is changed while the casting thereof is being continued.
This problem becomes important particularly during a casting process, i.e., when the casting width is changed without interrupting the casting process. Namely, when the width of the metal being cast has been changed, it is also necessary to change simultaneously the range of cooling water located on the rear surface of the belts in the transverse direction thereof. To take a more positive measure, it is necessary to simultaneously change heating ranges at opposite side end portions of the belts, but this means has not yet been proposed.
Furthermore, it has been found that it is difficult to restrain the deformation of the belts by adjusting the temperature distribution of the belts in the transverse direction thereof and by heating the opposite side end portions thereof to such a extent that no adverse effect is exerted to the thin slab. The reason for this difficulty is that, in the temperature distribution of the belt, there are two temperature distributions in the width-wise direction and the thickness-wise direction of the belt, and although, with the proposed method described above, the temperature distribution in the transverse direction of the belt can be made uniform, the temperature distribution in the thickness-wise direction of the belt cannot be made uniform. In other words, according to the proposed method described above, it is possible to prevent the deformation in the transverse direction of the belt, but the deformation in the longitudinal direction of the belt cannot be prevented.
As a measure against this problem, U.S. patent application No. 3,878,883 (Japanese Patent Examined Publication No. 59-4225) discloses stretching the belts and imparting thereto tension of 8,000 to 20,000 pounds per square inch of the cross section of the belt.
However, it has been found that, with this method as well, it is impossible to sufficiently prevent the occurrence of displacement of the belts when casting is effected while changing the width of metal being cast.